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7. POWER PLANTS WITH FUEL CELLS7.1 INTRODUCTIONThe operating principles of fuel cells were demonstrated initially in 1839 at theRoyal Institution of London by an English barrister and physicist, Sir WilliamGrove, who showed the reversibility of water electrolysis. The first practical applicationof fuel cells is credited to Francis T. Bacon of Cambridge University. In 1950Bacon published groundbreaking results of an alkaline cell prototype. Fuel cellsthen became known worldwide when the National Aeronautics and Space Administration(NASA) used them in the Apollo program during the 1950s and later in theGemini program. Obviously, fuel cells were a very convenient technology for thespace program in not being polluting, producing electricity and heat, and having asby-product potable water from hydrogen, exactly what scientists wanted for aspaceship. In the past few years fuel cells have appeared as the most promisinginnovation in the market of alternative energies for stationary, portable, and automotiveapplications, as a natural energy conversion system from hydrogen storedfrom electrolysis. What appeals most about fuel cells is their construction, whichcan be clean and compact, their need for only a few movable parts, their modulartechnology, and the fact that they do not inflict on the environment emissions ofsulfur and nitrogen oxides (SOx and NOx) [1–3].Present interest in fuel cells is enormous. Numerous companies and researchcenters throughout the world are working on many developments related to fuelcell energy systems: Ballard Generation Systems, Global Thermoelectric, FuelCell Technologies (Canada), Sulzer Hexis (Switzerland), UTC Fuel Cells, SchatzEnergy Research Center and Energy Partners, M-C Power, General Motors,Siemens–Westinghouse Corporation, GE Power Systems, Teledyne Energy Systems,H-Power, Avista, Ida Tech/North West Power Systems, and Plug Power inthe United States; Toshiba, Mitsubishi Electric Corporation, and Ebara Corporationin Japan; ECN in Holland; Nuvera Fuel Cells in Italy; Rolls-Royce in England; andMTU, DaimlerBenz, Dornier, and Buderus Heiztechnik in Germany, among others.All this interest in fuel cells supports the idea that direct combustion of fossil fuel isdeclining in importance. Current efforts toward commercial and regular productionof fuel cells are intensive, primarily to improve its performance as related to thespace between electrodes. This space is critical for the compactness of this energysource, as well as for the removal of sulfur and carbon monoxide, particularly in thePEM (proton exchange membrane) and SOFC (solid oxide fuel cell) types. Thesecompounds contaminate the platinum catalysts, thus degrading fuel cell performancewith time.Fuel cell characteristics differ from those of current dominant technologies fordistributed generation in electric power systems, which are based on internal combustionengines using reciprocal primary movers or steam turbines. Such technologiesare widely used at present and offer cheap, reliable energy with satisfactoryheat use and efficiency. However, these types of machines cannot definitively findplace in a planet concerned with its own survival. Characteristics such as noise,vibration, and emission of pollutant gases (e.g., NOx, COx) have not yet foundthe most appropriate optimization formula of acceptance. Because of that, frequentmaintenance should be planned, thus increasing the cost of small units, whichalready have low overall efficiency, on the order of 30% [4–6]. Fuel cells seemto be a good option despite not yet having a mature enough technology to be a feasiblesolution for the world market.7.2 THE FUEL CELLFuel cells are electrochemical devices sometimes compared to conventionalautomobile batteries. However, there is a fundamental difference in that fueland oxidizer are supplied to fuel cells continually so that they can generate continuouselectric power. Therefore, a fuel cell is important as an energy vector,combining with the stored fuel as a potential energy source. Whereas fuelcells are based purely on the electrochemical reaction of hydrogen or other fuelgases, in ordinary batteries the reactants are either consumed or must be regeneratedthrough electric recharge, as in automotive lead–acid batteries [1,2]. Thehydrogen combines with the oxygen inside a combustion-free process, liberatingelectric power in a chemical reaction that is very sensitive to the operatingtemperature. Despite initial use in aerospace applications, new experimentalmaterials have been contributing significantly to improved use, efficiency, andcost of fuels for terrestrial applications. A very significant figure that shows alot of improvement is the amount of platinum catalyst needed in the electrodes.It has decreased in the last few years from 28 mg/cm2 of electrode area to about0.1mg/cm2 [3].There are different types of fuel cells classified according to their operation atlow or high temperatures. Low-temperature PEM fuel cells (up to about 100_C) areavailable and ready for mass production. They are used in applications where hightemperaturecells would not be suitable, such as in commercial and residentialpower sources as well as in electric vehicles. The most compact systems are appropriatefor powered electric vehicles and are available from 5 to 100 kW. High-temperature(about 1000_C) SOFCs are typically used for industrial and largecommercial applications and operate as decentralized stationary units of electricpower generation. Because of such high operating temperatures, great heat dissipationis expected to be recovered, and integration with gas-powered microturbines isvery successful. A 70% overall efficiency is reported in such applications, representinga great contrast with respect to the energy generation in coal power plants,whose efficiency is between 35 and 40%.Figure 7.1 shows the principles of an operating fuel cell. They are powered bythe flow of a fuel gas such as hydrogen, which could be pure or derived form(e.g., methanol, coal gas, natural gas, gasoline, naphtha) and that of an oxidizer(e.g., oxygen, air). After the hydrogen passed through a porous anode separatedby an electrolyte of a porous cathode, the resulting ion meets the oxygen to formwater. As a result of this reaction, the accumulated electrons in the anode form anelectrical field and find their way through an external conductive connectionbetween the anode and the cathode. An electrical current is established through anexternal load. As in any other chemical reaction, heat is produced which has to bedissipated in some way. As a result of the gas reaction, there is by-product generationof heat and water, in contrast with the conventional mixture and burning offossil fuel.From a physiochemical point of view, fuel cells are the electrolysis antipode processin the sense that the first consumes H2 and O2 and produces electricity, heat,and water, whereas the second consumes electricity and water and produces heat,H2, and O2. This duality widely motivates practical possibilities for fully clean andsustainable systems, as discussed in Chapter 2.7.3 COMMERCIAL TECHNOLOGIES FOR GENERATIONOF ELECTRICITYHydrogen and pure oxygen are not yet commercially available in the quantitiesrequired for cost-effective use in fuel cells. There is abundant oxygen in the airand abundant hydrogen in several organic materials, particularly natural gas. It isavailable at low temperature but demands a fuel processor, or reformer, beforereaching the cell. The reformer converts, methane, for example (added in asteam) to a reformed mixture of hydrogen, sulfur, ammonia, carbon dioxide,and carbon monoxide. The fuel processor still has to accomplish a catalyticendothermic steam reform: that is, the reaction of water steam with fuel hydrocarbonto produce hydrogen, thereby increasing complexity and losses, leadingto an efficiency of around 35 to 48%. Purification of hydrogen is still requiredbecause particulates may degrade the fuel cell membrane life. Although naturalgas is a fossil fuel, it can bridge the current economic status toward sustainableproduction of hydrogen within a couple of decades. One issue that needs to beevaluated is the cost comparison of microturbines powered by natural gas withfuel cells running from reformed hydrogen. The fuel cell choice makes senseonly when combined with other applications that are interconnected to hydrogenstorage and production. Therefore, fuel cells will be feasible economically whencombined with hydrogen-powered electric vehicles, electrolyzers, and smallscalerenewable energy sources.One of the peculiarities of fuel cells to be used as commercial sources of energyis the need to consider the benefits and restrictions of a very complex market concernedwith the following four points:Cost: Projected at $2 to $3 per watt, the cost of fuel cells is much higher than thecost of generators, batteries, and other forms of alternative energy. A recentvery positive trend motivated by the manufacture of fuel cell–powered carsallows a projection of $0.79 per watt in full-scale production. Competitivenessis a little more difficult with river and maritime applications, equipmentfor telecommunications, and recreation vehicles, which already have areasonable market share [4–7].Product availability: Customers want to have products to try out and comparewith other options; otherwise, it becomes very difficult to convince people touse them.Fuel: With the exception of large boats that use diesel or gasoline, several otherindustry segments use propane. However, if hydrogen is available forautomobiles, which is a much larger market, this will easily be extended toboats.Safety: A certain apprehension exists on behalf of potential buyers of recreationvehicles and small boats about replacing propane with hydrogen.Power generation faces a highly competitive market under deregulation policies.Therefore, economic factors must convey a proper decision as to the type of generatorto be used for each application. The main economical factor is maximizationof the commercial power made available to consumers with minimum maintenanceat the highest levels of efficiency and minimum capital investment [4,6,7]. The onlyitem difficult to analyze in fuel cell applications is still a possibly lower capitalinvestment. Many countries, including Germany and England, have been investingeffectively in clean generation policies by subsidizing initial production and use offuel cells. For a market with such diversified demands, several fuel cell technologiesare in development [8–11]. The main research lines being considered at thetime of publication of this book are:_ Proton exchange membrane or solid polymer fuel cells (PEMFCs or SPFCs)_ Phosphoric acid fuel cells (PAFCs)_ Alkaline fuel cells (AFCs)_ Molten carbonate fuel cells (MCFCs)_ Solid oxide fuel cells (SOFCs)_ Direct methanol fuel cells (DMFCs)_ Reversible fuel cells (RFCs)Common application ranges for these fuel cells are shown in Figure 7.2.The designation of a fuel cell type is related to the electrolyte for the reactantions. For example, in a proton exchange membrane (PEM), a solid polymer (SPFC)such as Nafion (a registered trademark of Dupont) is used to enable conversion ofhydrogen chemical energy (H2) and an oxidizer, oxygen (air or O2), into electricalenergy. Nafion operates at temperatures between 45 and 100_C with a typical efficiencyof 40%. This solid polymer membrane is the basis for the strongest interestin its use with automotive applications. The main function of such a polymer is toprovide good electronic insulation and an efficient gas barrier between the twoelectrodes while allowing rapid proton transport and high current densities. Byits consistency, the solid electrolyte does not move with respect to the electrodes.It does not diffuse or evaporate but occupies a moderate space with appreciableweight. These cells are promising for automotive applications and are expectedTo make fuel cells as light and small as possible, one has to construct compact modulesfor easy and efficient setup. This is the case for the PEM polymeric membrane,bonded on each side by catalyzed porous electrodes. Such an electrode–electrolyte–anode or membrane electrode assembly (MEA) is made with bipolar electrodeplates so that the positive side of one electrode plate is the negative side of thenext plate. Therefore, the MEA is extremely thin, compact, and ready to be associatedwith others. Furthermore, the electrode plate pair is typically machined withflow fields for even distribution of fuel and air or oxygen to anode and cathode. Itmust also be provided with a compatible path for the cooling air or water at theback of the reactant flow field of the metallic electrodes. The humidity must bekept in the approximate range of 85 to 100%, as the membrane hydration assistsin proton conduction through the electrodes. Thus, a small section of the MEAhas to be set aside for humidification of the reactant gases. High standards forthe MEA ohmic, mass transport, and kinetics characteristics are essential for itscommercial application.The phosphoric acid fuel cell (PAFC) has a certain similarity with the PEMFC,but the acid H3PO4 is the electrolyte and the cell is operated with hydrogen and air.The power density produced by this fuel cell ranges from about 0.20 to 0.35 W/cm2.The temperature range is about 220_C, with a typical efficiency of 40%. This cell istolerant to poisoning up to 1% of carbon monoxide, although good water managementis essential in its operation to improve reagent kinetics as dependent on temperaturesvarying from 150 to 220_C. The internal environment becomes verycorrosive, due the high electrode potential of its operation, demanding the use ofstrong, corrosion-resistant materials in its construction. This type of fuel cellachieves only moderate current density. Its construction and chemical reactionsare quite similar to those of the PEMFC, from which most of its features werederived. Because the temperature range of the PAFC is higher than that of thePEMFC, the PAFC is recommended for combined heat and power applications.There are many 200-kW units installed all over the world based on this technologywhich have a strong record of operation. The chemical reaction at the anode of aPAFC is similar to that in a PEMFC [3,5]:The alkaline fuel cell (AFC) was the first workable power unit used by NASAin the manned Apollo spaceship mission. Nevertheless, it was soon found thatcost, reliability, ruggedness, safety, and ease of AFC operation could never competewith some of the other technologies known at that time, although it had theleast expensive construction technology. It operates in temperatures between 55and 120_C with a typical efficiency of 50%. Its major problem is that stronglyalkaline electrolytes such as KOH and NaOH adsorb CO2 and so reduce itselectrolyte conductivity. This feature limits its current densities. The chemicalreaction at the anode of an alkaline fuel cell isThe electrolyte of a molten carbonate fuel cell (MCFC) is a mixture of alkalicarbonates of potassium and lithium. A carbonate ion moves from cathode toanode, where in combination with hydrogen it forms water and carbon dioxideplus the electrical charge. The power density produced by this fuel cell ranges toabout 0.10 W/cm2, limited by ohmic losses. The high operating temperatures(around 650_C) and the high corrosivity of the molten carbonate salts demand specialtypes of manufacturing materials, but these are justified by the noticeableimprovement in reactant kinetics and reduced need for expensive noble catalysts.The typical efficiency is above 50%. The bipolar electrodes are made from highqualitystainless steel protected by additional coatings of nickel or chrome. Thehigh temperatures allow for internal fuel steam reforming, such as the gas methane.Its high operating temperatures recommend MCFC primarily for stationary powerapplications in the megawatt range. The chemical reaction at the anode of a moltencarbonate fuel cell is [3,5]The solid oxide fuel cell (SOFC) uses yttria and zirconia as oxides to operate athigh temperatures (around 1000_C). Under such conditions, hydrogen and ionizedoxygen are combined to form water in the anode and to liberate a pair of electronswith excellent reactant kinetics, although the cell’s reversible potential is a littlelower for lower-temperature cells. There is no water management problem becausethis fuel cell has an entirely solid-state construction and a single side tube sealing.This cell may use hydrogen or carbon monoxide as fuel with efficiencies higherthan 50%. Its main drawback for high power generation is that of the materialsrequired in its construction. Recent research developments involve three main typesof reactant core design for this cell: (1) planar (similar to that of other fuel cells),which has sealing problems; (2) tubular, to improve the electronic conductivity andovercome the sealing limitations of the planar design; and (3) flattened tube, withbetter air guidance, to improve the packing densities of the tubular design. In anycase, the tube itself forms the air electrode. The power density produced by the tubulardesign cells ranges from about 0.18 to 0.20 W/cm2. The planar design cell mayreach a power density of about 0.35 W/cm2. SOFC use is restricted almost entirelyto stand-alone and combined heat and power applications. The chemical reaction atthe anode of a solid oxide fuel cell is [5]In the direct methanol fuel cell (DMFC), methanol is oxidized electrochemicallyby water at the anode to produce carbon dioxide and positive and negative ions, incontrast to what happens in a PEMFC, where the positive ions from the hydrogenare supplied directly to the anode. To improve rejection of the carbon dioxide in aDMFC, an acid electrolyte is used; otherwise, insoluble carbonates may form in thealkaline electrolyte. Similar to what is depicted in Figure 7.1, the hydrogen ionsproduced at the anode permeate the polymer electrolyte in the direction of thecathode, where they react with oxygen from the air to produce water. The externalcircuit provides a path for the flow of electrons accumulated in the anode. Themost attractive feature of the DMFC for the transportation and portable-use industriesis that a fuel reformer is not required. As may be inferred from this explanation,direct methanol fuel cells have poorer performance in the anode, wherefinding more efficient catalysts is still a major problem. In several countries othertypes of alcohol are being experimented with, such as ethanol and benzol. Thepower density produced by direct methanol cells as reported by Newcastle Universityresearchers in England is about 0.20 W/cm2. Its overall efficiency doesnot go above 50%. The chemical reaction at the anode of a direct methanolfuel cell is [2,5]The reversible fuel cell (RFC) (also known as a regenerative or unitized fuelcell ) is a special class of fuel cells that produce electricity from hydrogen andoxygen but can be reversed and powered with electricity to produce hydrogenand oxygen, as depicted in Figure 7.3. The three key concepts of the reversibleor regenerative fuel cell are output power, run time, and recharge rate. The numberand size of the fuel cells in the stack determine the output power, which is dependentprimarily on the effective area of each electrode–membrane–electrode assembly.The run time is determined by the capacity of the hydrogen storage tank available.The recharge rate is determined by the output rate of the electrolyzer used to producethe hydrogen. An example of this is a PEM-fed uninterruptible power supplyunder development by Unigen as part of the U.S. Department of Energy’s StateEnergy Programs for developing RFC modules ready for use.Reverse fuel cells are also under development in many parts of the world. Theycan convert hydrogen directly from water using photovoltaic, hydro, or wind power.It has been observed that RFCs are capable of an energy density of about 450 Wh/kg, which is 10 times that of lead–acid batteries and more than twice that of currentchemical batteries. One of these systems is reported by GreenVolt Power Corp.Typically, their fuel cell splits water into its components hydrogen and oxygenfor use in a variety of industrial and transportation applications. The unit uses20% less energy in a significantly smaller unit size than that of water–alkali electrolyzerdevices, and requires only distilled water to produce 99.5% pure hydrogenand oxygen. When used to power hydrogen fuel cells, the water by-product createdcan be reused by the RFC, potentially lowering operating costs. On-site hydrogen gasgeneration can also improve the economical constraint of hydrogen transportation bypipelines and bottles. It is possible to fabricate ceramic microtubes by electrophoreticdeposition (EPD), although further research into certain areas of this investigation isin progress.Figure 7.3 shows the chemical reactions in a reversible fuel cell. In the fuel cellmode a PEMFC combines hydrogen and oxygen to create electricity and water.When the cell reverses its operation to act as an electrolyzer, electricity and waterare combined to create oxygen and hydrogen. RFCs are expected to be usefulmostly in passenger cars, solar-powered aircraft, energy storage schemes, requiredpropulsion of satellites for orbit correction, microspacecraft, and power systems, asin load leveling in remote sources of wind turbines and solar cells.Yet another type of fuel cell is the zinc–air fuel cell, which has a gas diffusionelectrode, an anode separated by electrolyte, and mechanical separators. The gasdiffusion electrode is a permeable membrane that allows atmospheric oxygen topass through. After the oxygen has been converted into ions and water, the ionstravel through an electrolyte and reach the zinc anode. There it reacts with thezinc, forming zinc oxide, creating an electrical potential. This electrochemical processis very similar to that of the PEM fuel cell described above, but the refueling isvery different, although it has some of the characteristics of batteries. Zinc–air fuelcells are best suited for battery replacement but have another wide range of potentialapplications. Some developers of zinc-air fuel cells are Cinergy and MetallicPower and Electric Fuel Co.It is important to emphasize that each type of fuel cell has a different internal reaction,as illustrated in Figure 7.4.7.4.1 Low- and High-Temperature Fuel CellsThere are two broad categories of fuel cells: low-temperature or first-generation,which include AFCs and SPFCs (or PEMFCs) and the phosphoric acid type(PAFCs). The latter has been the main objective of resource investment to obtaina commercial model with compatible costs for terrestrial applications. High-temperaturecells such as MCFCs and SOFCs are considered second-generation, andtheir commercial development is at an advanced stage, being considered for usein large-scale electric power generation [1–5].Second-generation cells have been developed for operation at high temperaturesto couple easily to reforming of the hydrogen inside the cell. Such a proceduremakes the overall system very simple and increases the efficiency up to 60%, asdemonstrated under laboratory conditions using external fuel treatment.A single fuel cell can produce just a few watts. Therefore, they are stacked up toother cells, adding their effects electrically by series and/or parallel connection,very similar to ordinary batteries. A stack will then form autonomous modules ofenergy supply in accordance with the end purpose. There are currently very largepower stacks, up to 1.5MW.The electric power generated by a fuel cell stack is always direct current, just asordinary batteries. The dc power generated in the cells should be conditioned to thetypes of contemporary applications, as explained in Chapter 12. For all fuel celltypes there is a lot of packaging research aimed at turning them into useful commercialand practical products. The following characteristics are also the subject ofintense research:_ Pretreatment of fuel for cogeneration systems_ Heat recovery by preheating fuel and oxidizer_ Injection and recirculation of water or steam_ Energy administration, conditioning, conversion, and optimization_ Separation and recycling of CO2 in MCFCs by blowing air and carefuladministration of the water in SPFCs to assure that the membrane stays dampfor optimal conditions7.4.2 Commercial and Manufacturing IssuesThe Carnot cycle can be used to compare fuel cells with internal combustion engine(ICE) motors as generators of energy. This theoretical cycle imposes limits on theperformance and construction of any energy transformation unit by high pressuresand temperatures according to the upper efficiency limit Zc?(Thigh_Tlow)/Thigh.The dimensions of the heated machine also influence the overall efficiency directly,hopefully by about 30%. In the same way, theoretical limitations exist for fuel cells,although they may not be as narrow. The more pure the oxygen and hydrogen used,the higher the overall fuel cell efficiency rates, which are around 70%. Such rateswould be justified only in an aerospace mission, or in oceans or similar environments,where the costs of hydrogen and oxygen are not as relevant as in ordinarypower plants. Using methanol, for example, to obtain the fuel gas, the efficiencyfalls to around 44%; however, the cost reduction is dramatic. A practical alternativethat is perhaps already feasible is the use of SPFCs, as in the Gemini spaceship missionand as used for many years for the production of oxygen in submarines.One problem with fuel cells not solved completely when this book was written isrelated to how to obtain the largest possible contact area among electrolytes andgaseous reactants, called the notable surface action of the electrocatalytic conductor[1]. This conducting contact path involves the electrolyte itself, the floodedpores of the electrolyte, the platinum on the catalytic carbon particles, the polytetrafluorethylene(PTFE), and the gas through the pores.Power systems with fuel cells for remote applications (FCPS-RA) seems to bethe great opportunity for fuel cells in the current market. The power ranges between0.1 and 5kW would include loads such as recreation vehicles, communication stations,yachts, residences isolated from the public network, and canalizations ofnatural gas and oil. For these applications, the current retail prices for FCPSs are$0.13 (per kilowatthour) (generator), $0.29 (per kilowatthour) (battery), and $0.50(per kilowatthour) (photovoltaic panels) in the consuming markets. Current FCPSsin the wholesale can produce electricity for $0.44 (per kilowatthour). In the best ofhypotheses with a reduction of 50% in stack cost, an improvement of 100% in theperformance, and a hydrogen generation cost of $0.12 per gigajoule, FCPSs couldsupply energy at $0.23 (per kilowatthour).Commercial and manufacturing investment has been concentrated in three areas:(1) the development of a 5-kW reformer to be integrated with a fuel cell stack tolerantto fuel reforming; (2) commercially available fuel cells that operate with purehydrogen and are integrated with electric vehicles, and (3) diffusion and training inthe manufacture, production, and use of fuel cells.For small- and medium-energy systems, the present state of the art seems topoint toward the establishment to two already advanced technologies: protonexchange membrane and solid oxide fuel cells. Therefore, this chapter concentratesnext on those two technologies.7.5 CONSTRUCTIONAL FEATURES OF PROTON EXCHANGEMEMBRANE FUEL CELLSPorous separation between fuel and oxidizer is achieved by the use of specializedconventional materials (e.g., tissues made of carbon or carbon paper obtained fromTeflon) and is the most expensive part of the fuel cell stack. The main characteristicof these materials is that they should allow the flow of ions generated in the fuel andoxidizer reaction when going from anode to cathode, preventing the passage ofelectrons, so they have to circulate through an external circuit.Figure 7.1 is a simplified diagram of a fuel cell. At the anode side of the PEMfuel cell, fuel is supplied under a certain pressure, usually enough to make it gothrough the flow-field channels of the electrodes and cross the electrolyte. It isassumed for the sake of discussion that the fuel is the pure gas H2, although othergas compositions may be used, as discussed in Section 7.3. In these cases the hydrogenconcentration should be determined in the mixture. The fuel spreads throughthe electrode until it reaches the catalytic layer of the anode, where it reacts toform protons and electrons according to the reactionFigure 7.5 is a schematic diagram of a complete electrical energy generation systemusing a fuel cell stack with PEMFCs to be introduced in the market. The most vitalpart of the system is a stack with 70 PEM fuel cells and 300 cm2 per cell of activearea, supplying 3kW of alternating current through a dc–ac inverter. The diagramshows the stack fed with hydrogen and oxygen (air) as well as water for refrigerationand output products. Hot water and electricity are both available for the user.The overall stack output voltage is represented by Vs. Table 7.1 gives the main nominalcharacteristics of this system. The reformer for the cells to obtain hydrogenfrom a fuel with hydrocarbon and water steam is also represented. The capacityof the components in the system will depend primarily on the total output powerof the stack.Most of the head of series consist of a low-pressure, high-volume air blower tosupply the oxidizer. A water circulation system is used to cool the stack reactionsand to provide humidification of the input flows of air and reformer fluids. Thereformer is of the partial oxidation type, to generate fuel rich in hydrogen for thestack from the tank of natural gas or propane.A typical system setup consists of a fuel cell stack conditioned by a dc–dc converterof 48V to feed a blower, water pump, 4-kW power inverter, and anotherinverter to generate 120V ac output voltage. Auxiliary power can typically befed from dc–dc converters operating from the main fuel cell dc-link bus for microcontrollers,relays, solenoid valves, and so on.A microcontroller can be used to manage the fuel cell operation, providingacquisition of operating data and to inform the user about the stack operating conditions.A startup battery must be included to bootstrap the beginning of operations,the microcontroller, and other load controls. After fuel cell starting procedures(warm-up time), the system will be fully running only with power from the fuelcell stack. The PEMFC is recommended for either stationary or vehicle powerapplications and there are many 5- to 250-kW units based on this technology worldwidethat have had many hours of operation.7.6 CONSTRUCTIONAL FEATURES OF SOLID OXIDEFUEL CELLSSolid oxide fuel cells use an entirely solid oxide ion conducting ceramic of zirconia(zirconium oxide) stabilized with yttria (yttrium oxide) without any liquid-stateinteracting product. It works at very high temperatures, typically between 800and 1100_C. Therefore, there is no need for electrocatalysts, the most complexitem commonly associated with research in ceramics and membranes in all theother fuel cells. It may operate with hydrogen with some level of carbon monoxide,and as a result, CO2 recycling is not necessary. As a contrast to low- and mediumtemperaturefuel cells, the ions crossing the electrolyte from the cathode to theanode are the oxygen and by-product water formed at the anode side. At about800_C, zirconia allows conduction of oxygen ions starting up the energy productionprocess. The open-circuit voltage of SOFCs is usually lower than of MCFCs but incompensation, it has lower internal resistance, thinner electrolytes, and therefore,lower losses. For these reasons, SOFCs may operate at higher current densities(around 1000mA _ cm2).A zirconia mixture of ceramic and metal (cermet) is widely used to constructa highly resistant and stable anode for the high SOFC temperature environment.The metal used in this mixture is nickel because of its good electricalconductivity and catalyst properties, widening the operating range of this fuelcell since the fuel-reforming process can take place at lower temperatures. Onthe other hand, the cathode composition is still a complicated matter becauseof the cost of effective conducting materials at high temperatures. Some materialspresently used for this purpose are based on strontium-doped lanthanummanganite.The most challenging issues of such a slowly maturing technology are related tohigh-temperature-resistant materials, combined applications with other fuel cells,heat, and power management. At this stage of development it is difficult to predictwhich fuel cell technology is going to be the most suitable or which is going tobecome a successful commercial version that takes the best possibilities of heatand power combination (CHP) and the construction of hybrid systems with variousother fuel cell types. Figure 7.6 displays a possible system for SOFC cogeneration,including the electric conversion oxidizer input and fuel gases, heat exchanger, andcatalytic burner [11–14].There are other configurations considered seriously for a high-temperature combinationof SOFCs with steam or gas turbines (combined cycle system) where theexhaustion gases of the fuel cell would feed the gas turbine, which would power analternator. SOFCs of size 200 kW are being widely considered for large combinedheat and power generation units in shopping centers, hospitals, military headquarters,residential condominiums, public buildings, and stand-alone villages. In allthese applications, the natural gas has to be desulfurized before feeding the anode,and air is admitted into the fuel cell through preheaters using exhausted anode andcathode hot gases.7.7 WATER, AIR, AND HEAT MANAGEMENTOptimum operation management of fuel cells is vital for maximum profits in thisenergy system in financial and energy use terms. The products to deal with arewater, air, and heat, which are closely related to fuel cells in three aspects: (1) asa by-product of chemical reactions, (2) in the cooling circulation fluid, and (3) forheat recovery. The FC heat affects especially the speed of electrochemical reactionsbecause it eases hydrogen extraction from hydrocarbonates and increases the overallefficiency of the fuel cell by means of heat recycling for both ambient and waterheat and/or for heating boilers used as primary energy sources for turbine drivers ofelectrical generators and for hot water storage. As the heat is produced locally, theFC generating units may have increased their overall efficiency through the use ofinternally produced electricity and heat. Local production of energy decreases thetransportation and distribution losses and alleviates the public network differentlyfrom what happens in concentrated power plants, usually located at very long distancesfrom consumers [3,5].The water resulting from chemical reactions in fuel cells has to be removedonly partially since its presence increases the conductivity of the electrolyte. Conversely,if there is too much water in the electrodes bonding the electrolyte, pore floodingwill cause difficulties for fuel permeation, and as a result, the concentration lossesmight be higher. A good relative humidity should be maintained between 85 and100% for a reasonable operating balance. In some small fuel cells the air goingthrough the cell may be the same for both oxygen reactions and moisture balance(stoichiometric feeding is usually at a rate higher than 2). For larger cells there areusually two independent air supplies, and the moisture may have to be complementedby an external vapor source. Use of these quantities may be expressed as [3]Either air heat from the cooling process or hot water from the chemical reactionsmay be used in heat exchangers for commercial, industrial, and residential purposesand for warming up vapor and/or air conditioning in domestic homes.7.8 LOAD CURVE PEAK SHAVING WITH FUEL CELLSThe strategies presented in this section allow specification of the power ratingfor a fuel cell (FC) stack system under the approach of imposing a load curveto be as flat as possible. FC sizing can be based on the maximal load curve flatness,limiting the maximum load peak or the amount of the necessary thermalenergy. Other methodologies may also be used for FC sizing, depending on operatingmode and how hydrogen is going to be produced. In principle, the FC canoperate at constant or variable power during a selected period, depending on theamount of hydrogen available. For each situation, a dedicated procedure shouldbe applied [15–17].7.8.1 Maximal Load Curve Flatness at Constant Output PowerThe methodology of maximum load curve flatness at constant output power allowscalculation under a set of constraints. The major consumer interest is to decreasedemand during the peak period so as to reduce the cost of the energy consumed.On the other side, from a utility company point of view, there is, in addition toreduction in the maximum demand, a possibility of increasing the efficiency ofthe distribution network, due, for example, to a decrease in overall losses. The optimizationrationale is based on limitations of load curve changes. Load can only bedecreased during the load peak period (say, three hours) and can increase with theuse of an electrolyzer only during nightly runs (say, a six-hour period). With thispurpose, flatness of the load curve can be quantified by a form factor defined forevery distribution transformer as [16]Current converters to feed electrolyzers are of the current source-controlledconverter (CSCC) types. However, they present special characteristics of voltageand current. The rectifier current capacity depends on the amount of gas flowrequired. In the same way, it is possible to determine the power of the electrolyzerfor production of hydrogen out of the peak period to obtain a factor kf as close aspossible to 1. The specific electrical power that can be obtained from a hydrogenfedFC is 26:6 Vc kWh/kg, where Vc is the output voltage across the terminalsof every single cell belonging to the FC stack [3,16]. This calculation uses thestandard FC electrochemical model discussed in Section 7.12. The hydrogendensity in the FC is 0:084 kg/m3 (NTP), so the amount of H2(m3) necessary togenerate the right amount of energy is determined in normal cubic meters (Nm3)fromFigure 7.7a represents an example of the daily load curve of a small village servedby an energy system with both a fuel cell and an electrolyzer operating under constantfull power during both peak hours and off-peak hours. The difference betweenthe two curves in the figure is the energy diverted to the electrolyzer from midnight to6 p.m. and the energy received by the load from the fuel cell power plant between 6and 9 p.m. The difference is due to the efficiency of the entire conversion loss.There are other possible modes of peak shaving (Figure 7.7b) when the period ofhydrogen production could be restricted to the morning hours. The fuel cell mayoperate at constant power or at variable power, so as to track as closely as possibleconsumer demand to keep the load curve at constant maximum demand.7.8.2 Amount of Thermal Energy NecessaryThe FC electrical efficiency may be determined fromThe stack heat losses can be separated into three categories: the rate of heatremoved by the water cooling system, Qw; the rate of heat loss in the air flowingout of the stack, Qa; and other losses, Qother (mostly stack heating and surface heatAn interesting case to be considered is water storage during the early hours ofthe day using the heat dissipated by the fuel cell between midnight and 6:00 p.m.An FC operating in this period allows enough heat dissipation in water for householdapplications such as those in the kitchen and bathroom in addition to air heating.The needs of a family can be accounted for on a per person basis i.e., assumingthat each person uses an average of 100 L/day of hot water, so a family with nppeople will need an average flow of 100np L/day [16]. Based on these data, thenecessary energy to heat up the required amount of water is calculated asAn interesting case to be considered is water storage during the early hours ofthe day using the heat dissipated by the fuel cell between midnight and 6:00 p.m.An FC operating in this period allows enough heat dissipation in water for householdapplications such as those in the kitchen and bathroom in addition to air heating.The needs of a family can be accounted for on a per person basis i.e., assumingthat each person uses an average of 100 L/day of hot water, so a family with nppeople will need an average flow of 100np L/day [16]. Based on these data, thenecessary energy to heat up the required amount of water is calculated asBased on the equations described in this section, it is possible to control the stackelectric efficiency to obtain more thermal or electrical energy according to the appliancedemand. Therefore, two operating modes may be selected for the fuel cell outputpower: variable power or constant power. Variable power aims at a constant load,and constant power aims at an optimal FC operating point for best operation andhydrogen consumption. In the general case, different operation regimes can be establishedbetween the storage and generation of energy. The electrolysis process can beused for these purposes in any off period or along shorter periods of time, in accordwith the needs of the utility or the consumer. To evaluate operation at variablepower, an iterative process has to be established. As already mentioned, due to lossesin the electrolysis and in power generation with FCs, the total daily consumption ofenergy will increase, although now shifted to a more convenient period.7.9 REFORMERS, ELECTROLYZER SYSTEMS,AND RELATED PRECAUTIONSThe main objective of using a reformer is to supply hydrogen locally to the fuel cellstack. It is a relatively complex system that demands automated control. The systemis sensitive to several aspects, such as control failure, the presence of inflammablegases, carbon monoxide poisoning gas, and overtemperature operation. An appropriatedesign should foresee several redundant sensors to guarantee safe operation,monitoring the components and controlling the stack operation. Although suchcomplex systems are usually encountered in industrial and automotive environments,a lot of research is still required for its use in efficient, safe, and reliablefuel cell systems.There are two major fluids present in the reformer: the fuel input and the hydrogengenerated. Precautions should be established for transportation, storage, distribution,and safety standards of the input fuel (methanol, propane, or natural gas).Safety requirements for hydrogen have already been adopted as related to smallscalestorage and proper pipelining, but it is expected that the fuel cell industrywill instill new practices with the growth of fuel cell applications.During reforming, small amounts of rusted carbon monoxide can be produced,but as long as there is no leakage, it does not represent a danger. However, if there isany leakage before the oxidation process, it is not safe and is a potential systemsafety concern. Reformers usually work at a temperature of 1650_C and must bethermally insulated to maintain a bearable exterior temperature. Internally, watersteam is used to cool the hydrogen. High internal temperatures can overheat andrupture the equipment. Leakage in the cooling area can produce steam jets athigh temperatures with eminent danger.Another way to obtain hydrogen is through renewable sources of energy. Theprocess to be used for that is the electrolysis of water or direct concentration of sunlighton reactors. This process can produce almost pure hydrogen and oxygen fedonly by secondary sources of energy, such as the surplus in power system installationsin the early hours of the day; Earth heat, such as geothermal, deserts, andocean temperature gradients; plus renewable sources of energy. Among renewablesources of energy with most promise for these purposes are hydroelectric powerplants such as those in Hamburg and Vancouver; biomass, such as sugarcane inBrazil; solar, geothermal, and wind, as in the Middle East; and sea tidal and seawave energy in almost every corner of the planet.7.10 ADVANTAGES AND DISADVANTAGES OF FUEL CELLSFuel cells are very appropriate for power plants using cogeneration systems in circumstanceswhere environmental issues such as a clean atmosphere, silence, andabsence of vibrations are of particular concern. This is because of the absence ofmovable parts other than the circulation pump and gaseous fluid blowers. PAFCs orcells using fuel reduction are not very convenient for power plant purposes becausethey are complex and necessitate specialized engineering services. All that is just tomaintain them safe and reliable, despite being smaller than those required for powerplants that use rotating machines.The modular nature of fuel cells allows their use in virtually any applicationthat allows flexible expansion of a power plant and gradual investment to followevolution of the loads. The energetic conversion factors are aided very favorablyby flexibility in the use of fuel, high efficiency (it may get near to 70%), and lowvolume/power ratio. In addition, the fuel hydrogen can be homemade using waterelectrolysis with solar energy or reforming technology producing hydrogen fromhydrocarbonate fuels.FC systems adapt easily for energy injection applications into the utility gridbecause they have energy density (Wh/m3) higher than that of standard batteries,relatively fast response to load fluctuation, high reliability, low cost of operation,and very low maintenance cost. Spilling of hydrogen will never be a major safetyconcern as this lighter-than-air gas flows up and away. When refueling fuel cells invehicles, a very fast time is possible, about the same as that to fuel an ordinary gasdrivencar, a very good advantage compared to recharging a battery.The disadvantages are the typical ones in any new technology that requires timeto mature and to be widely adopted. Fuel cell energy systems are unfortunately notcharacterized by historical data of reliability and continuous operation under harshconditions. Another drawback of fuel cell systems is the need for expensive noblematerials and susceptibilities to the contaminants present in the fuel.Hydrogen is considered to be the most efficient FC fuel, but it is not freely availablein nature. It has to be manufactured, and the full cycle of efficiency and costmust make sense. Therefore, the market for fuel cells is currently limited to specialapplications. On the other hand, hydrogen will not be widely available until there isan increase in demand with a corresponding economy of scale where the full cycleof efficiency sums up to a steady market. Although there are still some pessimistobjections to adoption of a hydrogen economy, it is expected that sooner or later itwill become a reality.7.11 FUEL CELL EQUIVALENT CIRCUITEvaluation of the dynamic performance of fuel cells for studies of electrical energygeneration systems is important to reduce cost and time at the design and testingstages. An electrical model may be derived from the electrochemical equations toenable determination of the open-circuit voltage and voltage drops of cells for a specifiedoperating point [17–21]. In power generation systems, the dynamic response is ofextreme importance for the control planner and system management, especially whenenergy is injected into the grid. So special attention has to be given to the dynamicresponse of FCs [22–24]. For energy injection into the grid, the generation controlhas to set the amount of power the FC will supply as a function of the load demand.As such, the dynamic FC response should be compatible with a fast variation in therandom load curve, which is not always the case, as we discuss next [16,21,22].From the reaction outlined in equations (7.1), (7.2) and (7.3), it is possible toobtain the electrical voltage generated in the electrochemical process in the cellif one recalls that two electrons pass through the external circuit for each watermolecule produced and each molecule of hydrogen present in the process. So electricalwork to move these charges is expressed aswhere F is the Faraday constant (or the electron charge in every molecules) and E isthe fuel cell voltage. If the system is reversible (no losses), the electrical work isequal to the Gibbs free energy released, which is the ‘‘energy available to do externalwork, neglecting any work done by changes in pressure and/or volume’’. Allthese forms of chemical energy are rather like ordinary potential energy withrespect to the zero-point energy and energy variation with respect to this point.The Gibbs energy is listed in the literature [1] for the reaction of water formationfrom 2H2 and O2 as in equation (7.3). For example, for a hydrogen cell operating at200_C, the Gibbs energy is _220 kJ, and from equation (7.24), E ?1.14 V.The activity of the reactants and products changes the Gibbs free energy of areaction. Balmer [17] showed in 1990 that temperature and pressure do affect thereaction activity, resulting in an electromotive force given in terms of the productand/or reactant activity, called the Nernst reversible voltage, ENernst.Therefore, reversible cell voltage is the cell potential obtained in an open-circuitthermodynamic balance (without load). In this section ENernst is calculated from amodified version of Nernst’s equation, with two extra terms to account for changesin temperature with respect to the standard reference temperature, 25_C, and 100kPa or 1.00 atm pressure [18–20], respectively. This is all given byUsing the foregoing standard temperature and pressure values for _G, _S, and Tref ,equation (7.25) can be simplified to [13–17]on the cathode catalytic interface (mol/cm3). The values used in equation (7.27) areset by theoretical equations with kinetic, thermodynamic, and electrochemicalfoundations [15–18].The ohmic overpotential results from the resistance to electron transfer in thecollecting plates and carbon electrodes, and the resistance to proton transfer inthe solid membrane. This resistance is essentially linear. In this model, a generalexpression for the resistance was defined to include all the important parametersof the membrane. The equivalent resistance of the membrane is then calculated bywhere 181:6/?c _ 0:634? is the specific resistivity (_ _ cm) at no load current and30_C, T is the absolute temperature of the cell (K), c is an adjustable parameterwith a possible maximum value of 23; and e4.18(T-303)/T is a temperature factorcorrection if the cell is not at 30_C. The parameter c is influenced by the membranepreparation procedure and is a function of the relative humidity and stoichiometricrate of the anode gas. It can have a value on the order of 14 under the ideal conditionsof 100% relative humidity, and values on the order of 22 and 23 have beenreported under oversaturated conditions.Equation (7.28) may be used to obtain the ohmic overpotential of the membraneresistance:where RC represents the resistance of the electrodes and contacts to the ion transferthrough the membrane (electrolyte), usually considered constant. The concentrationor mass transport affects the hydrogen and oxygen concentrations. This, in turn,causes a decrease in the partial pressures of these gases. Reduction in the pressureof oxygen and hydrogen depends on the electrical current and physical characteristicsof the system. To determine an equation for this voltage drop, it is defined asthe maximum current density, Jmax, under which the fuel is being used at the samemaximum supply rate. The current density cannot surpass this limit because the fuelcannot be supplied at a greater rate. Typical values for Jmax are in the range 1000 to1500mA/cm2. Thus, the voltage drop due to mass transport iswhere B is a constant depending on the cell and its operating state (V) and J is theactual current density of the cell electrode (A/cm2).Before setting up an equivalent circuit to represent the cell dynamics, it is interestingto examine in greater detail the phenomenon known as the charge doublelayer. Such a phenomenon normally exists on every contact between two differentmaterials, due to a charge accumulation on the opposite surfaces or charge transferfrom one to the other. The charge layer on both electrode–electrolyte interfaces (orclose to the interface) is the storage of electrical charges and energy; in this way itbehaves as an electrical capacitor. If the current changes, there will be some elapsedtime for the load (and its associated voltage) to decay (if the current decreases) or toincrease (if the current increases). Such a delay affects the activation and concentrationpotentials. It is important to point out that the ohmic overpotential is notaffected, since this has a linear relationship with the cell current through Ohm’slaw. Thus, a change in the current causes an immediate change in the ohmic voltagedrop. In this way it can be considered that a delay of first order exists due to theactivation and concentration voltages only. The associated time delay t?s? is theproduct [18–21]In broad terms, the capacitive effect assures the good dynamic performance ofthe cell, since the voltage moves smoothly to a new value in response to a changein the load current. The electrical output energy of the cell is linked to a certainload, such as the load represented in Figure 7.5. There is no restriction withrespect to load type as long as the power supplied by the stack is capable offeeding it, or if it does not represent starting motors and fast transient responseloads. For example, in systems for injection of energy into a distribution network,the load can be a dc–dc boost converter, followed by a dc–ac inverter, connectedto the public network through a transformer. In stand-alone systems, it can be apure resistive load (heating) or a resistive–inductive load (motor) [24–27]. In anycase, the average current density J?A/cm2? through the cell cross-sectional area isdefined aswhere Vfc is the cell output voltage for each operating condition (volts) and Pfc isthe corresponding power (watts). In Table 7.3 are listed values of the parametersdiscussed in this section for a typical Ballard Mark V fuel cell.The equations derived above in this section allow the construction of an equivalentcircuit model as shown in Figure 7.8 to represent FC dynamic behavior asgiven in many references [2,13,14,16–18]. Equation (7.26) represents the activesource ENernst. The membrane and contact ohmic losses are represented byRr ? Rm ? Rc [equation (7.33)]. The time delay given in equation (7.32) includesthe capacitor, C, to represent the double-charge-layer effects on the output voltageand current. This capacitance is very large (a few farads), since it is directly proportionalto the electrode area and the manufacturing material, and it is inversely proportionalto the electrode thickness, which is numerically very small (a fewnanometers).Typical values for the components used in the circuit model of Figure 7.8 mayvary widely with the size, construction, and type of fuel cell, but as an example, thefollowing parameters could be used: ENernst ? 1:48V and C ? 3F; Ra is a functionof ifc as given in equations (7.27) and (7.33).The first term appearing in equation (7.36) represents the open-circuit voltage ofthe FC (operating voltage without load); the last three terms represent voltage dropsto the net voltage of the cell, Vfc, at a certain operating current. The term ENernst isthe thermodynamic potential of the cell and represents its reversible voltage; Vact isthe voltage drop due to the anode and cathode activation over potential, a measureof the voltage drop associated with the electrodes; Vcon represents the voltage dropresulting from the concentration or mass transportation of oxygen and hydrogen(concentration over potential); and Vohmic is the ohmic voltage drop (ohmic overpotential), a measure of the ohmic losses associated with the conduction of protonsthrough the solid electrolyte and internal electronic resistances [8]. Many othermodels can be found in the literature. [16–22].Figure 7.9 shows the performance curves for a typical membrane used in fuelcells, which may be superimposed on the theoretical curves described by equations(7.35) and (7.36), respectively, for Pfc and Vfc. The efficiency Z is defined as therelationship between the electric output power and the energy corresponding tothe fuel input [3]. When the water product is in liquid form, it is usually given as7.12 PRACTICAL DETERMINATION OF THE EQUIVALENTMODEL PARAMETERSSeveral techniques have been used to obtain equivalent circuit parameters. A possiblemethod is the electrical impedance spectroscopy in which a variable-frequencyalternating current is applied through the cell, causing a voltage dropacross its terminals. The cell equivalent impedance is derived from the relationshipbetween this voltage and the applied alternating current. The frequencies used inthese tests may be as low as 10 mHz.Another very simple method used to obtain equivalent model parameters is thecurrent interruption technique. In this method, the concentration or mass transportovervoltage has to be neglected. Assume that a load is connected acrossthe FC terminals at VL (volts) and IL (amperes) represented in Figure 7.8. Aninterruption in the FC load current will cause a sudden voltage increase Vr acrossits terminals. This is measured through a data acquisition system or a storageoscilloscope to show that there is a sudden jump of voltage followed by a capacitorcharge–like response, as illustrated in Figure 7.10. Therefore, there is aneed not only for instantaneous switching but also for fast sampling of the voltageincrease to enable a clear separation of the activation overvoltage and theovervoltage due to the ohmic losses. This can be interpreted as if at the exactinstant of the current interruption the voltage across the charge-double-layercapacitance cannot immediately change [21–24]. After some time the FC outputvoltage would tend to Eoc ? VL ? Vr ? Va. A small data extrapolation may benecessary to intercept the vertical line of the exact instant when there was thecurrent interruption. As zero current is assumed to occur after the load current,there will only be capacitor discharge through Ra. The following expressionscould be used to express these changes:Notice that for a given electrode area, Ra is dependent on the load current, temperature,and specific resistivity. So the voltage across the double-layer capacitanceVt1 ? ILRa ? Eoc _ VL _ Vr at the very instant the load current was interruptedmay be considered as the discharge of a capacitor through the variable resistanceSeveral computer software products can be used for solving or simulatingelectrical circuits, such as MatLab/SIMULINK, PSim, or PSpice, with a computationflow as in the graph illustrated in Figure 7.12. In this calculation the internalvoltage drops are calculated from the fuel cell internal parameters measuredin laboratory tests. Either solutions suggested in Figure 7.13 could be used toobtain the voltage across the equivalent capacitance of the charge double layer[18,19]. The solution in Figure 7.12a is preferred since it is very easy to calculatethe initial value of ia assumed soon after any constant-state condition prior to thedesired calculation interval:7.13 ASPECTS OF HYDROGEN AS FUELHydrogen is the lightest and most buoyant element, so if it is released into an openspace, it disperses quickly, reducing dramatically the chance of ignition. In general,for every 96 parts of air, at least 4 parts must be hydrogen before there is a threat ofcombustion. This is actually quite a high concentration relative to other commonlyused fuels. For this reason it is good to know the conditions under which this happensbased on such properties as: ignition energy, ease of flotation, diffusivity,flammability limits in the air, and combustion energy.The combustion energy of the hydrogen is very low, which makes it even easierto catch fire, being in the ratio 1:10 for gasoline and 1:15 for natural gas or propane.All these gases are in fact of very low ignition energy, such that the probabilityof ignition of a mixture of any of them is relatively high even for a weakignition source. Comparable examples include mixtures of 4 to 75% of hydrogenin the air, 5 to 16% of natural gas in the air, and 1.4 to 7.6% of gasoline steam inthe air.Hydrogen possesses 2.4 times more combustion energy stored per unit of massthan does natural gas or gasoline. In volumetric terms hydrogen has much lessenergy: It has 25% of the explosion energy of natural gas and 0.3% of the liquidgasoline per unit of volume under normal conditions of temperature and pressure.The amount of energy stored in small systems of hydrogen is usually less than thatcorresponding to 4 L of gasoline.Advantages of hydrogen include its high diffusivity and flotation capacity. Suchproperties help to avoid fuel mixtures, and when this happens, they last for shortertimes. Hydrogen is four times more diffusive than natural gas and eight times morediffusive than gasoline.The first strategy used to prevent hydrogen from starting a fire is based onreducting the possibility of forming a combustible mixture, which can be accomplishedusing very well sealed canalization to avoid leakage. When one of thesehappens, the hydrogen will be dispersed quickly unless it is contained. The secondstrategy would be an ambient well ventilated to reduce or, in some cases, to eliminatethe area in which a combustible mixture may develop. In the case of leaks, thisreduces the exposure time to the possibility of developing an eventual combustiblemixture. The third strategy is the minimization of near-ignition sources such an asstatic discharges, open fires, hot surfaces (temperatures higher than 585_C), andother equipment able to produce sparks.The flammability of the hydrogen is the greatest danger in its use. However, theproblem may be limited by storage of only small volumes. The most dangerous andsusceptible parts refer to canalization for the gas, electrical equipment, the fuel cellitself, the control system, and the system reformer. A control system should be usedto establish safe operation of the power plant. If it extrapolates the safety limits, thecontrol system should turn off the power plant, switching alarms on and possiblyinforming operators regarding details of the dangerous conditions. The control systemshould also minimize dangerous situations due to single, multiple, or simultaneousfailure of components.The efficiency of a fuel cell is a function of the operating voltage of the entirestack. Higher voltages produce higher efficiency and therefore less consumption ofhydrogen per kilowatthour, but less output power is supplied. For a better trade-off,the choices for a given output power are between (1) operating at higher voltages,increasing the number of cells in the stack and thus the capital costs or (2) operatingat higher densities of current with fewer cells, higher fuel costs, and a shorter usefullife.Fuel cell and hydrogen costs, together with stack efficiency, will determine thefinal costs ($/kWh). The current technology, shown in Figure 7.9, displays theperformance of an individual cell belonging to a PEM stack with 70 cells, providingof 33% efficiency in generating electric power at a cost of $0.23/kWh based on ahydrogen cost of $12/GJ. A higher fuel cost would require a stack with more cellsoperating at higher efficiency to minimize the total cost.7.14 FUTURE PERSPECTIVESSince fuel cell principles were discovered more than one and half centuries ago by alayperson and continue to be studied today in sophisticated laboratories, fuel celltechnology is about to become mature and to change the way our society handlesthe production, storage, and delivery of energy. Enormous investments are beingcommitted to the development of a reliable and workable energy system able toreplace the fossil fuel model in current use. The benefits of such a breakthroughwould be to decrease pollution in urban areas, reduce greenhouse gas emissions,and increase the energy independence of oil-consuming countries. But hydrogencannot be compared directly with fossil energies, because it is only an energy vector,not an energy source. As such, it simply makes it possible to transmit a givenquantity of energy from the place of production to the place of consumption.As a result of current social and economic conflicts such as the continuing profitabilityof fossil fuel providers, it is not perhaps lack of technological knowledgethat causes the greatest difficulty but selection of the right moment to begin thetransforming process. Automobile manufacturers have been very aggressive andinvesting massively in developing not only fuel cell systems but all the requiredperipherals for easy integration to household use.The most important barriers to development of fuel cell applications, besidescost, are the absence of production infrastructure and distribution networks, aswell as the difficulties encountered in developing hydrogen storage technologies.Therefore, use of hydrogen in the transport sector should remain relatively limitedin the short run. On the other hand, the production of electricity (stationary fuelcells) and the storage of energy for mobile equipment (cell phones, laptops, etc.)are expected to be commercially available at competitive costs in the very nearfuture.The expected worldwide shift in the political power balance, new commercialdevelopments, environmental concerns, and energetic solutions are being consolidated.A new equilibrium point will eventually be found for a future era that willprovide the same level of comfort, lifestyle, and power strategies or better thanthose of our present society. ................
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